An air conditioning system such as a multi-zone air conditioning system for buildings is basically composed of one heat source unit, i.e., an outdoor unit and indoor units each provided in a plurality of spaces to be air-conditioned and connected to the outdoor unit, to control the temperature of each space to be air-conditioned. Conventionally, there has been an air conditioning system for cooling or heating interior spaces to be air-conditioned by circulating a refrigerant, such as hydrofluorocarbon (HFC), between the outdoor unit installed outside a building and the indoor units provided in the interior spaces of the building and by transferring, through the indoor units, heat from/to the refrigerant heated/cooled in the outdoor unit to/from the air in the interior spaces to be air-conditioned. In the system, the refrigerant might leak out in the spaces to be air-conditioned because of its structure adapted to circulate the refrigerant to the indoor units.
For that reason, there has been an air conditioning system in which a relay unit is provided between the outdoor unit and the indoor units to circulate a refrigerant from the outdoor unit to the relay unit and to circulate a heat medium, such as water, from the relay unit to the indoor units, and heat exchange is performed between the refrigerant and the heat medium through intermediate heat exchangers provided in the relay unit. The relay unit is further provided with heat-medium flow-path switching devices for sending the heat medium heated by one of the intermediate heat exchanger (hereinafter, referred to as a heating-use heat medium) to indoor units that are running in heating operations and for sending the heat medium cooled by the other intermediate heat exchangers (hereinafter, referred to as a cooling-use heat medium) to indoor units that are running in cooling operations. And the heated heat medium and the cooled heat medium are heat-exchanged, in the respective relevant indoor units, with the air in the interior spaces to be air-conditioned. Thus, a cooling operation or a heating operation can be performed for individual interior spaces to be air-conditioned (see, for example, Patent Document 1).